We have been engaged in the purification of Met- and Lys-tRNA synthetases from rabbit liver. Yeast tRNA was utilized in the enzyme assays. Some years ago we observed that the enzyme activities dropped to very low levels unless an additional material was added to the assay solution. We have now established that this factor is a protein that somehow increases the capability of yeast tRNA to accept amino acids in the xynthetase reactions. We call this material tRNA activator. This protein apparently is also present in many samples of isolated mammalian tRNA unless it has been repeatedly extracted with phenol or chromatographed. We are in process of attempting to (a) purify this activator protein, and (b) explain its role in aminoacyl-tRNA synthetase reactions. At some point we plan to return to the original problem of purification of one or more of the aminoacyl-tRNA synthetases. There are reports that four or five of these synthetases, including Met and Lys enzymes, occur in a complex. Other reports, however, do not support the idea of a complex. We hope to resolve this inconsistency.